The use of chemical compositions as catalysts in reactions which involve reaction conditions of high stress, (such as high temperatures and pressures) requires a catalyst composition which is highly resistant to abrasion due to the deleterious effects of reaction conditions. For example, neat (i.e., unsupported) vanadium tetroxide has been used to catalyze the vapor phase oxidation of lower aliphatic hydrocarbons such as butane to acetic acid, but this "neat" catalyst lacks the physical strength and abrasion resistance requirements of a commercially acceptable catalyst.
This neat catalyst also lacks sufficient heat transfer characteristics for the highly exothermic vapor phase oxidation. Accordingly, hot spots are produced in the catalyst bed during the vapor phase oxidation.
In addition, when using neat vanadium tetroxide in the above reaction, it is necessary to maintain a careful balance between temperature and the ratio of lower aliphatic hydrocarbon to oxygen in order to prevent the neat catalyst from being oxidized to vanadium pentoxide.
Another problem which occurs when neat vanadium tetroxide is used as a catalyst for the vapor phase oxidation of lower aliphatic hydrocarbons is that substantially all (i.e., more than 75 percent) of the total conversion occurs in the first portion of the catalyst zone or bed (i.e., the first 25 percent of the total catalyst zone or bed) which is in contact with the reactants. This concentration of conversion in the first portion of the zone in an exothermic reaction raises the exotherm temperature at that point substantially in excess of that at later points in the catalyst zone. These high exotherm temperatures make control of the reaction more difficult, dictate more expensive heat-resistant materials, often increase the yield of undesirable by-products, and are otherwise disadvantageous.
Supported catalysts have been suggested as a means of overcoming some of the above problems. For example, in U.S. Pat. No. 3,644,509, which is assigned to the assignee of the present invention, there is disclosed an advantageous method for preparing a supported metal oxide containing catalyst useful in the vapor phase oxidation of unsaturated aldehydes to the corresponding acids.
The search has continued for high efficiency catalysts of increased physical strength and abrasion resistance which are useful in reactions involving conditions of high stress.